Various arrangements have been proposed, when there are several telephones that may be connected to one line, to prevent any additional telephones other than the one in use from being connected to the same incoming line. The need for this arises in a number of different situations. In its simplest form, there may be a party line where there is a single incoming line which may be connected to any one of several telephones. A telephone normally has a "hook switch" or other main control switch which is closed when it is desired to talk. Unless some means is provided to prevent this, any other party "on the line" may listen in on the conversation by closure of the hook switch or its equivalent. Other situations that arise are those in which there are several incoming lines and a plurality of phones, any one of which may be connected to any one line. A typical case is the so-called key system. Here, where one party has answered or initiated a call and is using one of the lines, any other person may by, closing the hook switch or other main operating switch of another telephone, connect his telephone to the same line and listen to the conversation.
Various arrangements have been proposed for preventing the invasion of privacy that arises from a person picking up the receiver of another telephone and listening in on the conversation. In some cases these arrangements have involved intricate relay or switching arrangements at the key service unit in the building in which the telephones are located. It is desirable, however, to have an arrangement which may be readily applied to an existing telephone to insure its privacy. While such arrangements have been provided, they often have the weakness that it is difficult to get another party on the line if the using party desires this. In some cases, while a second party can be placed on the line, it is almost impossible to connect a third party to the line. Often, it is desirable that two or three people listen to a particular telephone conversation. Furthermore, in many cases, the presence of the privacy device makes it difficult to use the telephone as part of an intercommunication system. Obviously, the same privacy device which prevents a second party listening in on a conversation tends also to prevent someone from calling another party.